On the male trees most of the flowers include pistils, but
these likewise drop off at an early period; and the ovules, which ultimately
abort, are very small compared with those in female flowers of the same age.

Of the other or monoicous sub-group of polygamous plants, or those which bear
hermaphrodite, male and female flowers on the same individual, the common Maple
(Acer campestre) offers a good instance; but Lecoq states that some trees are
truly dioecious, and this shows how easily one state passes into another.
(Introduction/19. 'Geographie Botanique' tome 5 page 367.)

A considerable number of plants generally ranked as polygamous exist under only
two forms, namely, as hermaphrodites and females; and these may be called gyno-
dioecious, of which the common Thyme offers a good example. In my seventh
chapter I shall give some observations on plants of this nature. Other species,
for instance several kinds of Atriplex, bear on the same plant hermaphrodite and
female flowers; and these might be called gyno-monoecious, if a name were
desirable for them.

Again there are plants which produce hermaphrodite and male flowers on the same
individual, for instance, some species of Galium, Veratrum, etc.; and these
might be called andro-monoecious. If there exist plants, the individuals of
which consist of hermaphrodites and males, these might be distinguished as
andro-dioecious. But, after making inquiries from several botanists, I can hear
of no such cases. Lecoq, however, states, but without entering into full
details, that some plants of Caltha palustris produce only male flowers, and
that these live mingled with the hermaphrodites. (Introduction/20. 'Geographie
Botanique' tome 4 page 488.) The rarity of such cases as this last one is
remarkable, as the presence of hermaphrodite and male flowers on the same
individual is not an unusual occurrence; it would appear as if nature did not
think it worth while to devote a distinct individual to the production of
pollen, excepting when this was indispensably necessary, as in the case of
dioecious species.

I have now finished my brief sketch of the several cases, as far as known to me,
in which flowers differing in structure or in function are produced by the same
species of plant. Full details will be given in the following chapters with
respect to many of these plants. I will begin with the heterostyled, then pass
on to certain dioecious, sub-dioecious, and polygamous species, and end with the
cleistogamic. For the convenience of the reader, and to save space, the less
important cases and details have been printed in smaller type [].

I cannot close this Introduction without expressing my warm thanks to Dr. Hooker
for supplying me with specimens and for other aid; and to Mr. Thiselton Dyer and
Professor Oliver for giving me much information and other assistance. Professor
Asa Gray, also, has uniformly aided me in many ways. To Fritz Muller of St.
Catharina, in Brazil, I am indebted for many dried flowers of heterostyled
plants, often accompanied with valuable notes.

CHAPTER I.

HETEROSTYLED DIMORPHIC PLANTS: PRIMULACEAE.

Primula veris or the cowslip.
Differences in structure between the two forms.
Their degrees of fertility when legitimately and illegitimately united.
P. elatior, vulgaris, Sinensis, auricula, etc.
Summary on the fertility of the heterostyled species of Primula.
Homostyled species of Primula.
Hottonia palustris.
Androsace vitalliana.

It has long been known to botanists that the common cowslip (Primula veris,
Brit. Flora, var. officinalis, Lin.) exists under two forms, about equally
numerous, which obviously differ from each other in the length of their pistils
and stamens. (1/1. This fact, according to Von Mohl 'Botanische Zeitung' 1863
page 326, was first observed by Persoon in the year 1794.) This difference has
hitherto been looked at as a case of mere variability, but this view, as we
shall presently see, is far from the true one.